1. Field of the Invention
The present invention relates to a non-aqueous electrolyte lithium secondary battery, and particularly to an improvement in the negative electrode lithium.
2. Description of the Prior Art
Non-aqueous electrolyte rechargeable batteries generally comprise a negative electrode having an alkali metal such as lithium as its active material and an electrolyte obtained by dissolving a solute such as LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6 or LiCF3SO.sub.3 in an organic solvent such as propylene carbonate, .gamma.-butyrolactone, dimethoxyethane, tetrahydrofuran or dioxolan. Nowadays, since the non-aqueous electrolyte batteries have a high energy density, they are widely used as power sources in a variety of applications in small-sized electronic appliances such as electronic watches and clocks or cameras.
One of the problems encountered with the non-aqueous electrolyte batteries in enabling them to be rechargeable is that the alkali metal deposited on the surface of the negative electrode during the charging process is not flat but is in a state of tree-branch, needle and or fibril. In other words, a remarkable formation or growth of dendrite is observed with the deposited alkali metal. Once the dendrite in these states is formed, there will be produced not only a danger of an internal, short-circuit between the negative electrode and the positive electrode as a result of a growth of the dendrite but also a cuttings of the dendrite into pieces as a result of a local dissolution of the formed or grown dendrite, during the subsequent discharging process. Therefore, this produces a situation that all of the deposited alkali metal cannot be dissolved during the subsequent discharging process, thereby considerably deteriorating the charge/discharge efficiency of the battery.
In order to suppress the formation of the dendrite during the charging process, there has so far been proposed a method of charging the battery under such mild conditions as maintaining the negative electrode potential at -50 mV vs. Li.sup.+ /Li or more noble, wherein the alkali metal (lithium) is deposited at a quantity of electricity density lower than 0.3 mAh/cm.sup.2 (Progress in Batteries and Solar Cells, Vol. 2, pp. 54, 1979). Also, there has been proposed a method of improving the charge/discharge efficiency by combining a solvent having a high dielectric constant with another solvent having a low viscosity to be incorporated in the electrolyte (Electrochimica Acta, Vol. 30, pp. 1715, 1985).
In order to perform the charging process under the mild conditions as proposed in the above-mentioned publication, the area of the electrode plate must be expanded to 10 times or larger of that of the currently employed electrode, and thus, it is required to make the thickness of the electrode plate to be about 10 .mu.m, in point of view of the charging period of the batteries to be accepted by the demand of the current market.
Incidentally, the thickness of metal lithium foil currently employed is generally about 80-100 .mu.m, and the foil is prepared by means of extrusion molding through a die having a slit of the corresponding width by taking advantage of the softness of metal lithium. However, when it is intended to prepare the metal lithium foil having a thickness of about 10 .mu.m by this extrusion molding process, the metal lithium will deform in a wave form at the outlet of the slit, and thus, the preparation of an electrode of the desired size has been remarkably difficult.
Further, the metal lithium foil obtained by the extrusion molding process contains a multiplicity of crystal grain boundaries and crystalline defects, and the sizes of the crystal grains are widely varied or scattered. The metal lithium foil thus prepared therefore has a disadvantage that it sometimes reacts with an organic solvent contained in the electrolyte at the crystal grain boundaries which are chemically active, and the surface of the metal lithium is unevenly covered by a film of passivated state, thereby to promote the development of the dendrite and to deteriorate the charge/discharge efficiency.